TR2/3/3A Starter current/Bently

[TexasKnucklehead]

So after a trip to Austin last week, I have 800 miles on the TR3. It's running like a clock I and ran over to a friends to pick up a heater motor. When I start the car, it starts, but doesn't sound like the starter is happy. When I get home, and try another re-start, the starter refuses. I checked the voltages, and all is well up to the starter. So I pull the starter, see some oil and dirt on the commutator and clean it off. With a battery charger as a voltage source, it spins up slow, but seems fine. With the car battery, it seems fine too. (I love breaking down in my own garage.)

Here's where I get a little over involved. I send a note to some club members and it happens the monthly meeting is at our house the next afternoon. Suddenly I have 5 starters, not counting mine and I decide I need to measure the "light running current" as specified in the Bently manual. Starting Motor, Section 3, Performance Data specifies "light running current 4.5 amps at 7,400-8,500 rpm." I assume this would be the starter running without a load, and the "bench testing" method described would support this. However, the book recommends a meter capable of measuring 600 amps at 12v. I have 2 meters with 10aDC max, and another with 20aDC max -that peg when I try to read the "light running current". I added a length of 14g solid copper wire (with a typical resistance value of .003 ohm/ft) and can measure the voltage drop across the wire with the motor running at 200mV -with slight variance, all the starters are the same. So, if I=V/R I can assume the motors are drawing about 66amps.

My question is, why would the book suggest the current to be 4.5 amps? I know that assuming my foot of wire is exactly .003 ohms is probably not accurate, but I'm pretty sure my calculated value is closer to realistic than the book value.

My starter is back in the car, and it is starting fine, but I wonder about the accuracy of my test and the Bently.

 

[TR3driver]

I believe that is a misprint. The TR4 workshop manual (which is the same starter as TR3A after TS50k) gives 45 amps. 4.5 amps might be possible for a starter with a pure series-wound field, but the TR3 starter uses a series-shunt field that draws more than 4.5 amps even without the rest of the starter motor.

 

[TexasKnucklehead]

My neighbor just came back from cruising the coast and loaned me his DC current probe. The starters lying on the floor draw between 41 and 45amps. So, I suppose my foot of wire is really about .0044 ohms.

I suppose someone added a "." to the center of 45 in the original Bently, because I have a reprint with the same mistake. But I have renewed confidence in my rebuilt starter.

 

[TR3TR6]

After I rebuilt my engine, I noticed the starter didn't spin it as fast as before the rebuild. I had the starter rebuilt which helped, but it still wasn't as fast as before the engine rebuild. The problem turned out to be the engine was tighter after the rebuild. I ended up installing one of the high tork starters and have had no issues with it since.

 

[TR3driver]

I suppose someone added a "." to the center of 45 in the original Bently, because I have a reprint with the same mistake.

The Bentley manual is a reprint of the factory manuals anyway. I checked, and the 4.5 number does appear in later versions of the factory manual; but the first edition (without the TR3 supplement at the back) shows 45 amps. So I think we can blame S-T for the misprint, not Robert Bentley.

I'm pretty sure that Bentley just used a photocopy of the original manual anyway, they didn't even re-typeset the content. Which is why there are two set of page numbers; the original factory numbers (which started over for each chapter) and the Bentley numbers.